Compound drives are also called turbo-compounds. In such compound drives, the stream of exhaust gas from a combustion engine—in many cases a diesel engine—is routed through the exhaust gas turbine of a turbocharger. The charge air is fed to the combustion engine through a blower that is connected to the exhaust gas turbine with a rotationally fixed connection. As the exhaust gases continue on their way, in the existing art—in combination with a compound drive—they pass through a second turbine. This second turbine may transmit its rotational energy to a reduction gear, which is then connected in turn to a Föttinger coupling. After the Föttinger coupling there can again be a reduction gear, in order to further reduce the speed of rotation. Finally, the rotational energy is led into the crankshaft or into the centrifugal mass of the combustion engine. Through such a design, the energy content of the exhaust gases is used to increase the drive energy of the combustion engine.
Non-uniformities of rotation between the crankshaft and the turbocharger are evened out through the use of the Föttinger coupling. Otherwise a rigid power train of the composite drive would convey non-uniformities of rotation of the crankshaft all the way to the power turbine, which would lead to significant torsion vibration problems.
The design of a Föttinger coupling is very complex, which also makes this solution very costly. Furthermore, the efficiency is not optimal, due to the slippage inherent in the system.